gpg-interface: refactor 'enum sign_mode' parsing

gpg-interface: refactor 'enum sign_mode' parsing

The definition of 'enum sign_mode' as well as its parsing code are in
"builtin/fast-export.c". This was fine because `git fast-export` was the
only command with '--signed-tags=<mode>' or '--signed-commits=<mode>'
options.

In a following commit, we are going to add a similar option to `git
fast-import`, which will be simpler, easier and cleaner if we can reuse
the 'enum sign_mode' defintion and parsing code.

So let's move that definition and parsing code from
"builtin/fast-export.c" to "gpg-interface.{c,h}".

While at it, let's fix a small indentation issue with the arguments of
parse_opt_sign_mode().

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>